Question

In how many ways can a teacher arrange 6 students in the front row of a classroom with a total of 20 students?

Answer 1

I think that is example for variations without repetitions:

so count is 20*19*18*17*16*15=27907200 (multiplication principle)

See also:

http://www.hackmath.net/en/calculator/combinations-and-permutations?n=20&k=6&order=1&repeat=0